tapetum

from The Collaborative International Dictionary of English v.0.48

Tapetum \Ta*pe"tum\, n. [NL., from L. tapete a carpet, a
   tapestry.] (Anat.)
   An area in the pigmented layer of the choroid coat of the eye
   in many animals, which has an iridescent or metallic luster
   and helps to make the eye visible in the dark. Sometimes
   applied to the whole layer of pigmented epithelium of the
   choroid.
   [1913 Webster]